The present invention is directed to the art of game balls, and more specifically to the art of game ball construction. The invention is particularly directed to the production of softballs and baseballs having improved thermoplastic molded cores, and will be described with particular reference thereto. It will be appreciated, however, that the invention can have usefulness in other fields and applications and may be advantageously employed in other environments.
The present invention relates generally to improved core, or center, compositions for the construction of game balls such as softballs and baseballs. While the invention will be described with specific reference to traditional flight 12 inch two-piece molded softballs, it is understood by those skilled in the art that the invention may also be utilized for the construction of molded softballs and/or game balls of various sizes (i.e. baseballs are generally 9 inches in circumference, while softballs range in size from 10 to 16 inches in circumference). Furthermore, if desired, the core composition may be molded as a one-piece unitary structure, although the two-piece molded ball is preferred.
Traditional flight two-piece molded softballs can be formed according to processes and methods set forth in U.S. Pat. Nos. 4,725,058, 4,729,566 and 4,840,378. These softballs are generally comprised of a molded core and a molded cover. Although the dimensions and the properties of the molded balls differ depending upon league rules, playing characteristics desired, etc., the balls typically have a circumference of approximately 11 inches (i.e. an "11 inch ball") to 12 inches (i.e. a "twelve inch ball") and weigh about 160 to about 200 grams.
Specifically, according to the American Softball Association (A.S.A.) specifications, the weight of finished 11 inch balls ranges from 160-175 grams, and that of finished 12 inch balls ranges from 175-200 grams. The coefficient of restitution (C.O.R.) for traditional flight softballs is 0.500 maximum at 88 fps for both 11 and 12 inch balls. In addition, the compression of the traditional flight balls ranges between 0.007 and 0.014 inches.
The core, or center, of a two-piece molded 12 inch (30.5 cm) softball generally has a density from about 15.5 to 16.5 lbs. per cubic foot, and a hardness from about 72 to 78 on the Shore A scale. The core is typically formed from a cellular polymeric material, and can be comprised of polyurethane foam made up of an isocyanate and a polyol (see U.S. Pat. Nos. 4,725,058 and 4,840,378), or a foamed thermoplastic (e.g., ionomer resins or ethylene vinyl acetate (EVA), see U.S. Pat. No. 4,364,565). While its diameter is typically between about 3.65 and about 3.7 inches, and its weight ranges from about 135 grams to about 150 grams, the size of the core may be adjusted to compensate for the thickness of cover material chosen.
In this regard, the core of a 12 inch two-piece molded softball is generally surrounded by a cover having a specific gravity of about 1.2, a thickness of from about 0.035 to 0.060 inches (more typically between about 0.040 and 0.060 inches) and a hardness of from about 78 to about 90 on the Shore A scale. If a vinyl resin cover is utilized, the cover can be formed from a vinyl resin, a phthalate compound plasticizer, stabilizers and a titanium dioxide dispersion. It is typically molded over the core according to a plastisol (a dispersion of finely divided resin in a plasticizer) fusion process.
As a result of the strong impact and frictional forces normally applied during play to the finished softballs and baseballs, strong adhesive and durability properties are required by the cover and core compositions. Numerous compositions and processes have been developed for the purpose of improving the overall durability and playability characteristics of the balls.
One problem with the molded softballs of the prior art stems from the application of the cover to the core. Quite often, the thermoplastic resin cores do not withstand the cover molding cycle. For example, many of the core compositions of the prior art collapse or distort upon being subjected to the cover molding plastisol fusion process and are thus deficient in durability.
As a result of the deformation which occurs to the core during the process of cover molding, it has become desirable to develop a new core composition. Such a new core composition should have all the desirable playability properties of softballs but should also be made to withstand the collapsing or distortion which is common to cores upon molding of the cover.
In accordance with the present invention, thermoplastic core compositions are produced which are sufficiently durable to withstand the high temperatures and conditions produced during the cover molding cycle. In addition, the resultant game balls produced incorporating the core compositions of the present invention exhibit the durability and playability characteristics necessary for repetitive play.